High frequency induction heating equipment is more advantageous than a generally used atmospheric furnace in that the former allows operation to be done in a clean environment and a small lot of products to be processed in a short period of time efficiently. Generally when steel is induction quenched using an induction coil, power control is adopted to control a condition of a heat treatment of an object such that the power input to the coil and time are used as parameters. The steel is quenched under a variety of heat treatment conditions and the quenched object is confirmed in quality to empirically set an appropriate heat treatment condition. In this case whenever the object is changed the current heat treatment condition(s) must be reconsidered. Determining a heat treatment condition is thus cumbersome. This is attributed to the fact that it is difficult to adopt temperature control to control a condition of a heat treatment of an object by employing temperature and time as parameters.
When controlling a quenched object in quality, metallographically in particular, is considered, it is preferable that the object is quenched with temperature control. In induction quenching, however, it is technologically difficult to measure and rapidly control temperature. As such, quenching with temperature control is currently not adopted. It is difficult to measure temperature in an induction heating treatment because in contrast to an object heated in an atmosphere and thus treated, an object to be treated is directly heated and accordingly the measurement of temperature must be obtained directly from the object. Furthermore, high frequency induction heating equipment is often provided with a drive mechanism to drive an object to heat the object uniformly. This renders it difficult to lay out a contact type thermometer for installation and hence measure temperature.
This may be addressed for example by employing a radiation thermometer or a similar non contact type thermometer to facilitate measuring temperature. However, conventional radiation thermometers respond slowly and are thus unsuitable for measuring the temperature of metal. Thus there has not been a radiation thermometer suitable for temperature control in induction hardening. Recently, there have been provided radiation thermometers outputting signals faster and settable in emissivity to measure temperature with higher precision. This may find a possibility of employing a radiation thermometer in induction hardening to rapidly control temperature.
If induction quenching can be done with temperature control, induction heating, which basically, partially heats an object to be treated, does not provide the object with a uniform profile in temperature. As a result the object may have some quenched portion(s) different in quality. This is problematic when induction quenching is applied to a through hardening process. In particular, if the object to be treated is large in thickness, it would have a significantly uneven profile in temperature and thus be prone to have some portion(s) different in quality. If the object is not heated uniformly, the object has a portion sufficiently heated and hence satisfying a level of quality as desired as a product, and a portion insufficiently heated and hence failing to satisfy the level of quality as desired as the product.
Such problem may be resolved by heating the object for a sufficient period of time to utilize heat conduction to allow the object to have internal, uniform temperature. The problem may also be resolved by employing a high frequency power supply of a relatively small frequency to cause a magnetic flux to enter the object deeply to heat the object uniformly. Theses approaches, however, have a common issue to be addressed, i.e., that how the sufficient period of time for heating the object should be determined.